Electronic flash intensity control circuits

ABSTRACT

Electronic flash intensity control circuits for controlling the light output of a discharge tube, comprising a first capacitor having a stored energy, a trigger circuit connected operatively to the discharge tube for causing the discharge tube to operate and thereby the discharge of said first capacitor to initiate, a series circuit of a resistor and the discharge tube connected in parallel with the first capacitor for consuming a part of the stored energy in the first capacitor, a semiconductor control element having an anode, a cathode and a gate and connected in parallel with the resistor for being rendered conductive at a desired time so as to control the output of the discharge tube, and means connected to the semiconductor control element for applying a control voltage to the gate thereof, wherein the terminal voltage of the resistor in the course of the discharge of the first capacitor is divided by a suitable dividing ratio, one of the divided voltages is applied between the cathode and the gate of the semiconductor control element, and the one divided voltage and the control voltage are compared with each other to cause the semiconductor control element to conduct at the desired time.

United States Patent [191 Iwata [54] ELECTRONIC FLASH INTENSITY CONTROL CIRCUITS [58] Field of Search ..315/151, 152, 154, 156, 159, 315/241R, 241 P, 245;95/ll.5 R,1O B;

[56] References Cited UNITED STATES PATENTS 3,502,943 3/1970 Wechsler ..l ..315/241 X 3,465,656 9/1969 Wick et al.... ..315/241 P 3,626,246 12/1971 Higuchi ..315/151 1 Feb. 13, 1973 Primary ExaminerRoy Lake Assistant ExaminerSiegfried H. Grimm Attorn.eyStevens, Davis, Miller & Mosher [57] ABSTRACT Electronic flash intensity control circuits for controlling the light output of a discharge tube, comprising a first capacitor having a stored energy, a trigger circuit connected operatively to the discharge tube for causing the discharge tube to operate and thereby the discharge of said first capacitor to initiate, a series circuit of a resistor and the discharge tube connected in parallel with the first capacitor for consuming a part of the stored energy in the first capacitor, a semiconductor control element having an anode, a cathode and a gate and connected in parallel with the resistor Y for being rendered conductive at a desired time so as to control the output of the discharge tube, and means connected to the semiconductor control element for applying a control voltage to the gate thereof, wherein the terminal voltage of the resistor in the course of the discharge of the first capacitor is divided by a suitable dividing ratio, one of the divided voltages is applied between the cathode and the gate of the semiconductor control element, and the one divided voltage and the control voltage are compared with each other to cause the semiconductor control element to conduct at the desired time.

TRIGGER CIRCUIT PAIENTEI] FEB I 3 I975 SHEET 2 OF 6 FIG. 2

TIME

TIME

TIME

TIME

PAIENIEBFEBI I 3,71 ,752

SHEET 3 BF 6 FIG. 4

VOLTAGE V omou u|m C SHORT DISTANCE TIME LIGHT OUTPUT 4 LONG DISTANCE TIME ' PATENIEBFEBI 3191a sum u or 6' FIG. 50

STORED VOLTAGE FIG. 5b

DISTANCE FIG. 6.

PATENTEU FEB 1 3191s SHEETS 0F 5 FIG. 7.

B LONG-DISTANCE A SHORT-DISTANCE SHORT DISTANCE SmSo Em:

LONG DlSTANCE TIME PATENTEIJ 31913 3.716.752

' sum 80F 6 FIG. 80

BRIGHTNESS STORED VOLTAGE FIG. 8b

OUTPUT VOLTAGE ELECTRONIC FLASH INTENSITY CONTROL CIRCUITS The present invention relates to an electronic flash apparatus and in particular an electronic flash intensity control circuit, so constructed that its light output can be readily changed, and wherein the stored energy in a main discharge capacitor is consumed by a circuit ele ment such as a resistor so that when the stored energy reaches a preset value a discharge tube connected to the main discharge capacitor is caused to radiate.

' For a more complete understanding of the nature and scope of the present invention, reference may be had by way of example to the following detailed descriptions of the preferred embodiments taken in conjunction with the accompanying drawings in which:

FIG. 1 is an electrical circuit diagram showing an embodiment of the electronic flash apparatus according to the present invention;

FIG. 2 is a characteristic diagram showing the relationship between the voltages at different points of the circuit of FIG. I and variations in the, light output;

FIG. 3 is an electrical circuit diagram showing another embodiment of the present invention;

FIG. 4 is a characteristic diagramshowing the relationship between the voltages at different points of the circuit of FIG. 3 and the light output;

FIGS. 5a and 5b are characteristic diagrams showing the relationship between the main discharge capacitor voltage and the brightness and between the distance and the brightness;

, FIG. 6 is an electrical circuit diagram showing a further embodiment of the present invention;

FIG. 7 is a characteristic diagram showing the relationships between the voltages at different points of the circuit of FIG. 6 and the light output;

FIGS. 8a and 8b are characteristic diagrams showing the relationships between the main discharge capacitor voltage and the brightness and between the distance and the brightness, and

FIG. 9 is an electrical circuit diagram showing a still further embodiment of the present invention in which a light pulse generating circuit and an energy discharge circuit are separately provided.

Referring to FIG. 1 there is shown one embodiment of the present invention comprising a circuit wherein a main discharge capacitor I is supplied with its energy from terminals 10 and 11, a series circuit comprising a xenon discharge tube 2 and a variable resistor 3 is connected in parallel with the main discharge capacitor 1 and a parallel circuit comprising a variable resistor 8 and a zener diode 9 is connected between one end a of p the xenon discharge tube 2 and one end of the variable resistor 3 through one end of a capacitor 4, the anode of a semiconductor control element such as a thyristor and a resistor 6, while the intermediate point of the variable resistor 8 is connected to the gate of the thyristor 5 and the cathode of the thyristor 5 is also connected along with the other end of the capacitor 4 to the intermediate point b of the variable resistor 3. The operation of the circuit of this embodiment will be explained hereunder.

When a trigger switch 7 is short-circuited, the xenon discharge tube 2 starts its operation to thereby initiate the discharging of the stored energy in the main discharge capacitor 1 by way of the variable resistor 3.

While the variable resistor 3 is provided to consume the energy discharged from the main discharge capacitor 1,

its value is set such that the quantity of electricity supplied to the xenon discharge tube 2 through the variable resistor 3 may not have any significant effect on the degree of exposure.

Consequently, the stored voltage across the main discharge capacitor 1 causes the stored energy in the main discharge capacitor 1 to be consumed in accordance with a decay characteristic such as that of the voltage, between the points b and 0 shown in FIG. 2, which corresponds to the time constant of the capacitive value of the main discharge capacitor 1 and the resistance value of the variable resistor 3. In the process of this action, however, if the thyristor 5 connected in parallel with the variable resistor 3 is caused to conduct, the energy from the main discharge capacitor 1 is supplied to the thyristor 5 whose internal resistance is low as compared with the variable resistor 3, so that substantially the whole energy is applied to the xenon discharge tube 2 thereby producing a light output corresponding to the stored voltage across the main discharge capacitor 1 which is present at the time that the thyristor 5 is turned on. Thus, it is possible to vary the light output as desired by controlling the time of conduction of the thyristor 5.

Assuming now that the intermediate point of the variable resistor 3 is properly adjusted for the purpose of step-down so that the output voltage between the points b and c is adjusted to be as shown at b-c in FIG. 2, this voltage is applied to the gate of the thyristor S in the negative direction and the difference between the two voltages across the points b and c and the points d and e is applied between the gate and the cathode of the thyristor 5. Thus, if the gate voltage of the thyristor 5 is selected to be 1 volt, then with the voltage between the points d and e being 6 volts as shown by a straight line A in FIG. 2 the thyristor 5 will conduct at the time that a time T is reached where the potential difference between the points b and c is 5 volts, thereby producing a light output shown at A in FIG. 2.

On the other hand, if the voltage between the points d and e is selected to be as shown by a straight line C in FIG. 2, then a light output as shown at C which is small as compared with the light output A will be produced when a time T is reached.

Consequently, by varying the position of the intermediate point of the variable resistor 8 to change the voltage between the points d and e, the light output may be adjusted as desired thereby making a voltage detection type adjustment of a light output possible. On the other hand, the system of the present invention is such that the thyristor 5 may be caused into conduction at any desired time in the course of the main discharge capacitor 1 being discharged with the resultant certain time difference between the instant the trigger circuit is short-circuited and the time that the full radiation takes place. According to the experiments conducted by the inventors, however, there was practically no problem in the actual use of the apparatus since the time constant would be 5 ms, that is, the delay time could be reduced to the order of 1/200 seconds if the capacitive value of the main discharge capacitor 1 and the resistance value of the variable. resistor ,3 were selected to be .500 microfarads and 10 ohms, respectively.

Furthermore, while such a delay time may be reduced, if desired, by making the resistance value of the variable resistor 3 smaller, this increases the quantity of electricity supplied to the xenon discharge tube 2 by way of the variable resistor 3 producing a light output as shown at D in FIG. 2. However, with a proper calibration such an adjustment may be readily effected almost without any problem.

The capacitor 4 is used to prevent a misoperation due to a noise pulse, while the resistor 6 and the diode 9 form a voltage regulator circuit for stabilizing the voltage between the points d and e.

As described above, even in the case of a shortdistance photographing with a small light output, the stored voltage across the capacitor in the circuit construction of FIG. 1 is sufficiently high during the initial period when a trigger is applied so that the xenon discharge tube can be readily caused to radiate and the light output can be adjusted to a considerable degree. Moreover, an electronic flash apparatus can be provided which is very simple in construction and of a high practical value.

FIG. 3 illustrates another embodiment of the present invention whose circuit construction is such that the circuit comprises: a flash output control circuit composed of a xenon discharge tube 23 connected in parallel with a main discharge capacitor 21 and including a trigger circuit consisting of a trigger circuit 24 and a trigger switch 25; a capacitor 27 and the anode and cathode of a control element 28 such as a thyristor connected in parallel or across an intermediate point b and the other end a of a variable resistor 26; and a light-sensitive control circuit composed ofa capacitor 33 and a zener diode 32 connected in parallel with a light detecting circuit including a photoconductive element 34 such as a-CdS element, a diode 35 and a capacitor 36 whose both ends are connected between the base and the emitter of a semiconductor element 31 from the collector of which is connected a variable resistor 30 to one end of the zener diode 32 by way of the anode of the thyristor 28 and a resistor 29.

The operation of the circuit described above will be explained hereunder.

With the resistance of the variable resistor 26 being selected to be sufficiently large as compared with the internal resistance of a capacitor 22, the flash produced by the xenon discharge tube 23 will be controlled by the stored charge in the capacitor 22, and according to the present invention the value of the capacitor 22 is selected to be such that it may have no considerable effect on the exposure of an object to be photographed, so that it is utilized as a light source for finding the distance of the object from the light source producing a light pulse as shown at A in FIG. 4. The resistor 26 is provided to consume the energy from the main discharge capacitor 21 and the value of the resistor 26 I is so selected that the quantity of electricity supplied to .mined by the capacitive value of the main discharge capacitor 21 and the resistance value of the resistor 26, while in the process of this operation the reflected light from the object of a range-finding light pulse is received by the photoconductive element 34 so that the resistance value of the photoconductive element 34 decreases in accordance with the range-finding light pulse with the result that the capacitor 36 is caused to store through the diode 35 the energy from the constant voltage source comprising the zener diode 32 and the capacitor 33. Since this voltage is the one which is stored corresponding to the distance between the object and the light source, it represents the output voltage proportional to the inverse square of the distance as shown in FIG. 5b. On the other hand, the stored voltage across the main discharge capacitor 21 varies in such a relationship that the change therein is proportional to the square of the brightness shown in FIG. 5a. Thus, the combination of the two voltages makes a proportional light control possible, that is, an automatic adjustment may be effected by means of a linear function.

According to the system of the present invention, the stored voltage across the capacitor 36 causes the transistor 31 having a high input impedance to change the resistance between the collector and the emitter thereof. For example, when the distance between the object and the light source is short, the stored voltage across the storage capacitor 36 increases and hence the resistance between the collector and the emitter of the transistor 31 decreases, so that the voltage between the points d and e takes the form shown at E in FIG. 4.

On the other hand, the terminal voltage of the resistor 26 is substantially equal with the stored voltage across the main discharge capacitor 21 and so if the intermediate point b of the variable resistor 26 is properly adjusted to step down the voltage so that the voltage between the points b and c is adjusted to be as shown at G in FIG. 4, this voltage is applied to the gate of the thyristor 28 in the negative direction and thus the difference between the voltages across the points b and c and the points d and e respectively is applied between the gate and the cathode of the thyristor 28. Assuming now that the gate voltage of the thyristor 28 is 1 volt, since the voltage between the points d and e is 2 volts as previously mentioned the thyristor 28 will be ready for gating when a time T is reached and the voltage between the points b and 0 becomes -l volt, and then the thyristor 28 will conduct.

With the thyristor 28 now conducting, the energy from the main discharge capacitor 21 is supplied through the anode and cathode section of the thyristor 28 and through a portion (between the points b and c) of the variable resistor 26. in this case, the forward internal resistance of the thyristor 28 is very low and the resistance value of the aforesaid portion of the variable resistor 26 is very small as compared with the total resistance value of the variable resistor 26 since it has been stepped down as previously mentioned, and so the over-all internal resistance is practically zero resistance. Thus, the remaining energy of the main discharge capacitor 21 in the course of its discharge is almost entirely applied to the xenon discharge tube 23 so that it is discharged by the xenon discharge tube 23 as a light output for exposure suited to photographing. By this time the voltage across the main discharge capacitor 21 has discharged via the resistor 26 for a rather long time and consequently the light output in this case is very small as shown at C in the second curve from the bottom in FIG. 4. On the other hand, with a long distance between the object and the light source the stored voltage across the storage capacitor 36 is low and this increases the resistance between the collector and the emitter of the transistor 31 and the voltage between the points d and e takes the form as shown by a straight line F in FIG. 4. Consequently, by virtue of the potential difference between this voltage and the voltage between the ,points b and c the thyristor 28 conducts at a point when a time T is reached, so that since the stored voltage across the main discharge capacitor 21 is high as compared with that of a short-distance photographing a flash output as shown at D in FIG. 4 can be produced to provide a large light to the object thereby accomplishing the required automatic adjustment of the light output. Furthermore, since every operation of the light output adjusting circuit of the present invention takes place only after the conduction of the xenon discharge tube 23 there is a feature of less likelihood that any misoperation can occur, while the variable resistor 30 is provided for the purpose of sensitivity adjustment which may be accomplished by adjusting the voltage between the points d and e. However, since the system of the present invention is essentially such that a light pulse is produced to find out a range of an object from the light source and its reflected light is then stored in the storage capacitor 36 so that the thyristor 28 may be caused to conduct at any given time in the process of discharge of the main discharge capacitor 21 by means of the stored voltage across the storage capacitor 36, a certain time difference may result between the instant the range-finding light pulse is produced and the time that a full radiation takes place. According to the previously mentioned experiments, however, this time difference can hardly give rise to any problem in the actual use of the apparatus since the time constant of the capacitor and the resistor is l0 ms.

lt is now evident from the foregoing that the circuit constructed as shown in FIG. 3 has an important fea ture in that it eliminates complexity of the adjustment employing a transient integration characteristic and it also makes the automatic adjustment of a light output possible by means of a relatively simple circuit construction.

The circuit illustrated in FIG. 6 represents a further embodiment of the present invention whose construction will be explained hereunder along with its operation.

When a trigger switch 49 is short-circuited, a pilot discharge tube 47 supplied with the stored energy in a trigger capacitor 51 through a trigger transformer 48 starts to flash and the discharge of the energy from a main discharge capacitor 41 is initiated through a resistor 54 and a capacitor 53.

Then, if the value of the variable resistor 54 is selected'to'be sufficiently large as compared with the internal resistanceof the capacitor 53, the flash of light from the pilot discharge tube 47 will be controlled by the stored charge in the capacitor 53. According to the present invention, a light pulse such as shown at C in FIG. 7 is produced by setting the capacity of the capacitor 53 such that it can have no substantial effect on the exposureof an object, or alternatively a suitable means such as a filter and the gas in the discharge is employed so that the pilot discharge tube 47 may be utilized as a light source for light pulses to find the range of an object from the light source whose wavelength is outside the wave length sensitivity of photographic films. On the other hand, the resistor 54 is provided to consume the energy from the main discharge capacitor 41 so that the stored voltage across the main discharge capacitor 41 decreases in accordance with a decay characteristic which corresponds to the time constant determined by the capactive value of the main discharge capacitor 41 and the resistance value of the resistor 54. In the process of this decay, the reflected light from the object of a rangefinding light pulse is received by a photoconductive element 62 such as a CdS element, so that the resistance value of the photoconductive element 62 decreases according to the range-finding light pulse produced by the pilot discharge tube 47 and thus the energy is stored in a storage capacitor 61 through a diode 63 from a voltage regulator circuit comprising a voltage regulating element 59.

This voltage represents one which is stored corresponding to the range of the object from the light source and it possesses a voltage characteristic which follows the inverse square law of distance as shown in FIG. 8b. On the other'hand, the stored voltage across the main discharge capacitor 41 changes in a squarelaw relation with the brightness as shown in FIG; 80. Thus, with the combination of the two output voltages a highly improved controllability can be achieved by means of a proportional light control, i.e., through the use of a linear function.

According to the present invention, the resistance between the collector and the emitter of a semiconductor 60 having a high input impedance changes dependent upon the stored voltage across the storage capaci tor 61. Thus, if the distance of an object from the light source is short, for example, the stored voltage across the storage capacitor 61 increases and hence the resistance between the collector and the emitter of the semiconductor element 60 decreases, so that the voltage between points (1 and e in FIG. 6 (the voltage between the gate of a thyristor 56 and the emitter of the semiconductor element 60) takes the form as shown by A in FlG. 7. On the other hand, the terminal voltage of the resistor 54 is substantially identical with the stored voltage across the main discharge capacitor 41. Thus, assuming that an intermediate point b of the resistor 54 is suitably adjusted to obtain a step-down so that the output voltage between the points b and c may be adjusted as shown at F in FIG. 7, this voltage is applied to the gate of the thyristor 56 in the negative direction and thus the difference voltage of the two voltages across the points d and e and across the points b and c is applied between the gate and the cathode of the thyristor 56. Consequently, if the thyristor 56 is set such that it operates at the gate voltage of 1 volt, the thyristor 56 will be ready to conduct when the voltage between the points d and e is 2 volts and the voltage between the points b and c is l volt, that is, when a time T is reached after the generation of a light pulse C, whereupon the thyristor 56 will conduct so that a main discharge tube 44 is caused to discharge by means of a trigger circuit comprising a capacitor 46 and a trigger transformer 45. By this time, the terminal voltage of the main discharge capacitor 41 could be discharged considerably through the resistor 54 according to the curve shown at F in FIG. 7, and thus a light output produced will be very small.

On the other hand, with a long distance between the object and the light source the stored voltage across the storage capacitor 61 decreases with the resultant increase in the resistance between the collector and the emitter of the transistor 60, so that the voltage between the points d and e is as shown at B in FIG. 7 and the voltage between the points b and c is as shown at F in FIG. 7. Thus, by virtue of the potential difference between the two voltages the thyristor 56 will conduct when a time T in FIG. 7 is reached, and the stored voltage across the main discharge capacitor 41 is higher than that of a short-distance photographing. In this way, a large light output as shown at E in FIG. 7 can be applied to the object thereby accomplishing the required automatic adjustment of the light output.

However, since the light output control system of the present invention is such that the control is effected in the course of discharge of the stored voltage across the main discharge capacitor 41 by way of the resistor 54,

there inevitably results a delay between the instant a light pulse is produced and the time that a full radiation I occurs with the delay corresponding with the time of constant of the capacitor and the resistor may be made shorter, although there is possibility that the light from the discharge tube 47 may have an effect on the light for exposure of an object, Consequently, if the quality of light from the pilot discharge tube 47 is so chosen that it is outside the wavelength sensitivity of photographic films, it is theoretically possible to reduce the time constant of the main discharge capacitor 41 and the resistor 54, particularly the resistance value of the resistor 54 and it is thus possible to provide the auto.- matic adjustment of a light output practically involving no delay time.

Furthermore, since the light output adjusting circuit of the present invention is adapted such that it is rendered operative only after the main discharge capacitor 41 has started to discharge its energy by virtue of the operation of the pilot discharge tube 47, there is a distinguishing feature in that there is no possibility of any misoperation even if a light pulse is applied to the photoconductive element 62 before a synchronous-switch 49 is short-circuited, thereby ensuring a very stable operation. The diode 63 is used to prevent the energy stored in the storage capacitor 61 from flowing backward, while the capacitor 43 and the diode 42 are provided to store in the capacitor 43 the voltage developed across the main discharge capacitor 41 during its charging so that reverse current may be prevented by the diode 42 even if the voltage across the main discharge capacitor 41 is decreased, because the present invention employs a control system which controls the stored voltage across the main discharge capacitor 41 and thus there is possibility that the stored voltage across the main discharge capacitor 41 may drop below the starting voltage of the main discharge tube 44. ln this way, a stable triggering of the main discharge tube 44 is ensured.

FIG. 9 illustrates a circuit in which the generation of a light pulse and the discharge of the energy in the main discharge capacitor 41 are separately performed, that is, the light pulse is produced by means of a pilot discharge tube 47 as is the case with the embodiment of FIG. 6, while the discharge of the energy is effected by means of a control element 66 such as a thyristor. This circuit construction is advantageous in that the magnitude of a light pulse can be adjusted as desired.

What is claimed is:

1. An electronic flash intensity control circuit for controlling the light output of a discharge tube, comprising: a discharge tube and a resistive impedance connected together as a series circuit; a storage capacitor connected in parallel with said series circuit; trigger circuit means connected to said discharge tube for discharging said discharge tube and to thereby discharge said storage capacitor at least partially through said resistive impedance; a voltage-dividing tap on said resistive impedance for dividing a voltage applied across said resistive impedance by the discharge of said storage capacitor; control means for controlling the output of said discharge tube, including a semiconductor control element having an anode, a cathode and a gate and connected in parallel with said resistive impedance, said voltage-dividing tap being connected to said cathode for applying a voltage on the voltagedividing tap to the cathode; and control voltage applying means connected to said semiconductor control element for applying a control voltage to the gate thereof; wherein said semiconductor control element becomes conductive when the difference between the voltage on said voltage dividing means and said control voltage reaches a predetermined value.

2. The electronic flash intensity control circuit according to claim 1, further comprising: a second capacitor connected in parallel with said resistive impedance, a portion of the discharge of said storage capacitor being applied across said resistive impedance and said second capacitor at the start of the discharge of said storage capacitor to cause said discharge tube to produce a light output to be directed to an object; light signal storage circuit means connected to said control voltage applying means for storing a representative voltage which is representative of the distance between said discharge tube and said object, a voltage derived from said representative voltage being applied to the gate of said semiconductor control element as said control voltage; and light responsive means for applying at least a portion of said light output reflected from said object to said light signal storage circuit means.

3. The electronic flash intensity control circuit according to claim 2, wherein a power source for said light signal storage circuit means comprises the voltage applied across said resistive impedance.

4. The electronic flash intensity control circuit according to claim 2, wherein said control voltage applying means includes a transistor, said light signal storage circuit means is connected to said transistor for controlling the current flowing through said transistor by said representative voltage, and 'the output of said transistor is connected to said control means, wherein said control voltage applied to the gate of said semiconductor control element varies with said representative voltage.

5. An electronic flash intensity control circuit for controlling the light output of a flash discharge tube, comprising: a first discharge tube; a first storage capacitor connected in parallel with said first discharge tube; a series circuit comprising a second discharge tube in series with a parallel circuit of a resistor and a second capacitor, said series circuit being connected in parallel with said first capacitor; means to discharge said first capacitor at least partially through said series circuit to produce a light output from said second tube which is directed to an object; voltage-dividing means connected to said resistor for dividing the voltage applied across said resistor;trigger circuit means connected to said first discharge tube for generating a trigger pulse to discharge said first tube; control means controlling the operation of said trigger circuit means, said control means including asemicon'ductor control element, having an anode, a cathode and a gate, connected to said trigger circuit means and said resistor, said cathode being connected to said voltage-dividing means; control voltage applying means connected to said semiconductor control element for applying a control voltage to the gate thereof to cause said semiconductor control element to conduct and trigger said trigger circuit means; light signal storage circuit means connected to said control voltage applying means for storing a representative voltage which is representative of the distance between said pilot discharge tube and said object; light responsive means applying at least a portion of said light output reflected from said object to said light signal storage circuit means; and means applying a voltage derived from said representative voltage to the gate of said semiconductor control element as said control voltage, wherein said semiconductor control element becomes conductive when the voltage between said voltage dividing means and said control voltage applying means reaches a predetermined value.

6. The electronic flash intensity control circuit according to claim 5, wherein said means applying a voltage derived from said representative voltage comprises a transistor having its input connected to said light signal storage circuit means and its output connected to said control means. 

1. An electronic flash intensity control circuit for controlling the light output of a discharge tube, comprising: a discharge tube and a resistive impedance connected together as a series circuit; a storage capacitor connected in parallel with said series circuit; trigger circuit means connected to said discharge tube for discharging said discharge tube and to thereby discharge said storage capacitor at least partially through said resistive impedance; a voltage-dividing tap on said resistive impedance for dividing a voltage applied across said resistive impedance by the discharge of said storage capacitor; control means for controlling the output of said discharge tube, including a semiconductor control element having an anodE, a cathode and a gate and connected in parallel with said resistive impedance, said voltage-dividing tap being connected to said cathode for applying a voltage on the voltage-dividing tap to the cathode; and control voltage applying means connected to said semiconductor control element for applying a control voltage to the gate thereof; wherein said semiconductor control element becomes conductive when the difference between the voltage on said voltage dividing means and said control voltage reaches a predetermined value.
 1. An electronic flash intensity control circuit for controlling the light output of a discharge tube, comprising: a discharge tube and a resistive impedance connected together as a series circuit; a storage capacitor connected in parallel with said series circuit; trigger circuit means connected to said discharge tube for discharging said discharge tube and to thereby discharge said storage capacitor at least partially through said resistive impedance; a voltage-dividing tap on said resistive impedance for dividing a voltage applied across said resistive impedance by the discharge of said storage capacitor; control means for controlling the output of said discharge tube, including a semiconductor control element having an anodE, a cathode and a gate and connected in parallel with said resistive impedance, said voltage-dividing tap being connected to said cathode for applying a voltage on the voltage-dividing tap to the cathode; and control voltage applying means connected to said semiconductor control element for applying a control voltage to the gate thereof; wherein said semiconductor control element becomes conductive when the difference between the voltage on said voltage dividing means and said control voltage reaches a predetermined value.
 2. The electronic flash intensity control circuit according to claim 1, further comprising: a second capacitor connected in parallel with said resistive impedance, a portion of the discharge of said storage capacitor being applied across said resistive impedance and said second capacitor at the start of the discharge of said storage capacitor to cause said discharge tube to produce a light output to be directed to an object; light signal storage circuit means connected to said control voltage applying means for storing a representative voltage which is representative of the distance between said discharge tube and said object, a voltage derived from said representative voltage being applied to the gate of said semiconductor control element as said control voltage; and light responsive means for applying at least a portion of said light output reflected from said object to said light signal storage circuit means.
 3. The electronic flash intensity control circuit according to claim 2, wherein a power source for said light signal storage circuit means comprises the voltage applied across said resistive impedance.
 4. The electronic flash intensity control circuit according to claim 2, wherein said control voltage applying means includes a transistor, said light signal storage circuit means is connected to said transistor for controlling the current flowing through said transistor by said representative voltage, and the output of said transistor is connected to said control means, wherein said control voltage applied to the gate of said semiconductor control element varies with said representative voltage.
 5. An electronic flash intensity control circuit for controlling the light output of a flash discharge tube, comprising: a first discharge tube; a first storage capacitor connected in parallel with said first discharge tube; a series circuit comprising a second discharge tube in series with a parallel circuit of a resistor and a second capacitor, said series circuit being connected in parallel with said first capacitor; means to discharge said first capacitor at least partially through said series circuit to produce a light output from said second tube which is directed to an object; voltage-dividing means connected to said resistor for dividing the voltage applied across said resistor; trigger circuit means connected to said first discharge tube for generating a trigger pulse to discharge said first tube; control means controlling the operation of said trigger circuit means, said control means including a semiconductor control element, having an anode, a cathode and a gate, connected to said trigger circuit means and said resistor, said cathode being connected to said voltage-dividing means; control voltage applying means connected to said semiconductor control element for applying a control voltage to the gate thereof to cause said semiconductor control element to conduct and trigger said trigger circuit means; light signal storage circuit means connected to said control voltage applying means for storing a representative voltage which is representative of the distance between said pilot discharge tube and said object; light responsive means applying at least a portion of said light output reflected from said object to said light signal storage circuit means; and means applying a voltage derived from said representative voltage to the gate of said semiconductor control element as said control voltage, wherein said semiconductor control eLement becomes conductive when the voltage between said voltage dividing means and said control voltage applying means reaches a predetermined value. 